1. Field of the Invention
The present invention relates to a method for producing a ceramic heater exhibiting sufficient flexural strength, and not suffering fracture or similar damage in the course of production or use, and to a method for producing a glow plug incorporating the ceramic heater. The ceramic heater produced by the method of the present invention is useful as an element of the aforementioned glow plug employed for starting diesel engines and as a heating source employed in any of a variety of gas sensors, such as an oxygen sensor.
2. Description of the Related Art
Conventionally, a ceramic heater incorporating an insulative ceramic substrate (hereinafter also referred to as a “substrate”) and a heating resistor buried in the substrate has been employed for starting diesel engines or for quickly activating various sensors. The ceramic heater is used particularly in glow plugs or similar devices, whose temperature must be raised to 1,200° C. or higher. Many ceramic heaters have a structure such that a heating resistor is buried in an insulative ceramic substrate, wherein the heating resistor contains an electrically conductive sintered ceramic comprising WC or MoSi2, and the insulative ceramic substrate is formed of sintered silicon nitride ceramic and exhibits excellent corrosion resistance at high temperature. The insulative ceramic substrate comprises a pair of leads buried therein and made of a high-melting-point metal such as W for supplying power to the heating resistor (hereinafter also referred to as leads), wherein, for each lead, a first end is connected to a corresponding end of the heating resistor, and a second end is exposed on the surface of the substrate. Electricity is externally supplied through the leads to the heating resistor.
A conventionally known glow plug incorporating such a ceramic heater is generally configured such that a metallic outer sleeve surrounds the ceramic heater, and a metallic shell for mounting the glow plug on an engine surrounds the metallic outer sleeve. When a metallic sleeve is to be attached to a ceramic heater; more specifically, to a ceramic heater substrate formed of sintered silicon nitride ceramic, methods for attaching the metallic sleeve include a method wherein metal-ceramic joining is effected by use of an active brazing material. However, the above method is apt to result in variation in quality of the joined portions. In order to solve this problem, a joining method has been proposed which includes forming a glass layer on the heater, through baking, in order to enhance bonding of the brazing material to the heater, and then charging the brazing material into a space between the glass layer and the inner wall of the metallic sleeve.
The aforementioned conventional ceramic heater has a contact portion at which the substrate, the heating resistor, and the power supply leads, which differ in terms of physical properties (e.g., thermal expansion coefficient), are in contact with one another. Therefore, such difference in thermal expansion coefficient generates complex internal stress in the contact portion. The contact portion is the weakest portion of the ceramic heater. In the course of production or use of the ceramic heater, fracture is apt to occur at a certain part of a joint portion at which the heating resistor is joined to the power supply leads; i.e., a fitting portion, depending on the type of materials (e.g., ceramic) used to form the substrate and the heating resistor. Even when fracture is prevented, problematic cracking or a decrease in mechanical strength of the portion occurs. During the course of production of a glow plug incorporating the ceramic heater, when a metallic sleeve is fixed to the ceramic heater through brazing via a glass layer formed on the heater, a large stress is applied to contact portions at which the substrate, the heating resistor, and the power supply leads contact one another. Therefore, cracks may be generated at the contact portions between the heating resistor and the power supply leads, in the worst case leading to fracture of the ceramic heater.
In order to solve the above problems, for example, Japanese Patent Application Laid-Open (kokai) No. 7-282960 discloses a method including reducing stress concentration; specifically, rounding the tip of each lead, which is joined to one end of the heating resistor. Although the above method improves the structure of the ceramic heater, generation of complex stress cannot be completely prevented, the stress arising from differences in thermal expansion coefficient between the substrate, the heating resistor, and the leads. In addition, fracture or similar damage of the ceramic heater is not completely prevented. Furthermore, when the ceramic heater is assembled with a metallic sleeve, occurrence of problems such as fracture of the ceramic heater cannot always be prevented.